This invention relates to an autofocus system directed mainly to cameras, such as television cameras, which incorporate photoelectric conversion image sensors. The expression "autofocus" as used herein shall mean the function of an optical adjustment for automatically focussing images in an image-forming optical system without any recourse to human eyes.
The techniques heretofore known to the art as means for autofocus are basically classified as follows:
(a) Image definition detecting type PA0 (b) Distance measuring type
The definition of an image formed on the imaging plane or at a position optically equivalent to the imaging plane is detected and the image-forming optical system is adjusted in accordance with the resulting detection signal. As the signal for indicating the definition of the image, there may be used the value of differential of the image signal, the amplitude of the high-frequency component of the image signal, or the difference between the maximum and minimum values of the amplitude of the image signal. The techniques involving these signals are well known in the field of image processing.
The distance from the image-forming system (such as a lens) to the object is detected by a suitable method such as triangulation and the image-forming optical system is adjusted on the basis of this distance signal.
In the two types described above, the image definition detecting type is adopted in the system of the present invention. The reason for the choice of this type is (1) that for the focussing which is primarily aimed at obtaining a clear image of a given object, the detection of the image definition is most rational, and (2) that the distance measuring type tends to degradation in the accuracy of distance measurement with respect to objects at great distances or of cyclically repeating patterns.
FIG. 1 illustrates a conventionally known image definition detection type focussing system in a television camera. Such a system is disclosed in Japanese patent publication No. 5265/'64, for example. By a camera lens 11, the image of a given object is formed on an imaging plane of a camera tube of solid state image sensor plate 21 (hereinafter referred to as "image sensor"). The video signal output from the image sensor 21 is passed through a high-pass filter 22 capable of selectively passing a high-frequency signal component, a detector 27, and a signal processor 28 and applied to a driving power source 29. By the output from the driving power source 29, a drive unit 61 moves the lens 11.
The operating principle of this conventional system is as follows. When the image is correctly focussed, the image definition is maximized and, consequently, the high-pass component of the video signal is also maximized. Therefore, the focussing of the image is obtained by passing the output from the high-pass filter 22 through the detector 27 thereby measuring the amplitude of the output, and moving the lens so as to maximize the amplitude while judging the variation of the amplitude due to the movement of the lens by means of the signal processor 28.
The fault found with this conventional system is that when the position of the lens 11 is greatly separated from the correct focus position, namely when the image is blurred under heavy out-focus conditions, the amplitude of the high-pass component of the video signal becomes so small and the variation of the amplitude due to a small movement of the lens 11 also becomes so small that the out-focus signal cannot be supplied to the signal processor 28. This conventional system has another disadvantage that the system itself fails to follow a rapid movement of the object or the camera.